1. Field of the Invention
The present invention relates to a wire bonding method and apparatus for interconnecting electronic parts by using electrically conductive wires, and to a semiconductor device. Particularly, the inventive method and apparatus are applied suitably to semiconductor devices which are intended for high-speed switching of large currents in automobile equipment controllers, electric-car drive controllers and other vehicle-installed motor controllers.
2. Description of the Prior Art
In the manufacturing process of semiconductor devices which include multiple semiconductor chips and electronic parts, a scheme of wire bonding is used for the electrical connection between the electrodes of semiconductor chips and electronic parts and between the terminals of electronic parts.
A typical conventional wire bonding apparatus will be explained first with reference to FIG. 14. FIG. 14 is a side view of the conventional wire bonding apparatus. This wire bonding apparatus is designed to feed a wire 101, which is supplied from a bobbin (not shown), to the groove of wire press section 112 of a bonding tool 111 by way of a through-hole 115 formed in a horn 110 and a gap of clamp section of a wire clamp mechanism 120.
With ultrasonic vibration being applied to the bonding tool 111 which is fixed to the tip section of the horn 110, the wire 101 is pressed onto the electrode of a semiconductor chip 102 as one part of connection so that the wire 101 is joined to it, and next the wire 101 is fed and brought by the bonding tool 111 to the terminal 104 of another electronic part, e.g., a resistor, as another part of connection and joined to it in the same manner.
The wire clamp mechanism 120 is located between the wire press section 112 of the bonding tool 111 and the through-hole 115 of the horn 110, and it serves to hold and guide the wire 101 when it is fed out. The bonding tool 111 and horn 110 are supported on a vertical moving mechanism and horizontal moving table so that they can move vertically and horizontally relative to the semiconductor chip 102 and electronic part.
Automobile equipment controllers and electric-car drive controllers are required to be made much smaller in size and weight. The drive controller incorporates semiconductor devices which implements high-speed switching of large currents for producing a.c. power for driving motors by being supplied with power from such a d.c. power source as battery.
Electronic components have their operating currents increasing to match with the trend of higher-power drive controllers, and therefore wires of large diameters are used for the electrical connection between semiconductor chips and between semiconductor chips and electronic parts of semiconductor devices. For wires of large diameters, aluminum wires which are inexpensive and light are used, instead of wires having higher electrical conductivity that mainly consist of expensive gold. Aluminum wires are thicker due to the lower electrical conductivity than gold wires, and aluminum wires with diameters of 100-600 .mu.m are necessary for high-power semiconductor devices.
Semiconductor devices used in automobile equipment controllers and electric-car drive controllers are required to be durable against severe heat cycles and power cycles thereby to last long, in addition to the demand of compactness and light weight. In order to meet these requirements, it is necessary to improve the strength of wire joints.
There is a limit in widening the joint area based merely on pressing the wire 101 having a circular cross section onto the planar target joint surface, and there is also a limit in improving the strength and life of joints based merely on the application of ultrasonic vibration to the limited joint area. Specifically, the conventional wire bonding scheme works for joining by pressing the wire 101 having a circular cross section onto a planar target joint surface so that the wire is deformed, and the pressing force needs to be increased progressively to overcome the increasing resistance of deformation.
Accordingly, in order for the conventional wire bonding scheme to improve the strength and life of wire joints by raising the degree of deformation of the wire 101 while retaining the mechanical strength of the deformed section of the wire 101, it is necessary to increase the ultrasonic output for the metallic joint process thereby to increase the pressing force of the wire 101. However, an excessive pressing force by the increased ultrasonic output can result in the breakage of the electronic part or semiconductor chip 102 having the target joint surface.
On this account, conventionally, there is a limit in widening the joint area, and thus there is a limit in improving the strength and life of wire joints.